By 2030, global demand for energy is projected to increase by 50%. There is also growing awareness that the Earth's oil reserves may run out during the present century. Therefore, an energy shortage is likely to emerge, unless some renewable energy source replaces the fossil fuels in the next few decades. Fortunately, the Sun exposes the Earth with 10,000 times more power than the world's population consumes today. However, nourishing our civilization from this gigantic energy reservoir remains a challenge. For instance, at its maximum value of ˜1 kW/m2, solar irradiation incident on a car or plane is too low to power these vehicles. Therefore, for mobile applications, storage of solar energy in a fuel seems to be the inevitable choice. Indeed, by burning fossil fuels, we release the solar energy harnessed by biological photosynthesis millions of years ago.
Current solar panels may be utilized to charge batteries so that the solar power can be concentrated and stored. However, this presents its own set of challenges, including high startup cost and residual toxicity of materials used to create batteries. Additionally, chemical batteries have a limited service life and are relatively heavy for the amount of power they can provide relative to combustible fuels currently available. This results in limited range and usefulness of battery power for transportation use.
The hydrogen molecule, H2, is the cleanest and smallest carrier of chemical energy, yet it has the highest energy content per unit weight among all fuels: 52,000 Btu/lb, which is three times that of gasoline. H2 can be directly utilized as fuel in internal combustion engines for transportation, or for generation of electric power in fuel cells. H2O being the final product, no pollutants or greenhouse gases are produced. Indeed, hydrogen-powered buses and prototype H2-filling stations are already in service in Nagoya, Japan and Berlin, Germany.
Solar energy can be stored as chemical energy in H2 by photolysis of water: dissociation of H2O to H2 and O2 by photogenerated electron/hole pairs. In particular, significant research activity was stimulated towards photolytic cells producing hydrogen gas in 1972, when Fujishima and Honda demonstrated water could be split to hydrogen and oxygen under sunlight (photolysis) using an n-type titania electrode. At a solar intensity of 1 kW/m2 (maximum in the United States), and collector area of 1 mile square, a “photolysis farm” employing 10% conversion efficiency photolytic cells, should deliver H2 at a rate of more than 100 tons a minute.
Since the discovery of Fujishima and Honda (i.e., for almost 40 years), however, a stable photolytic device with an energy conversion efficiency of more than few percent could not be demonstrated. The realization of an efficient and stable photolytic energy conversion device is challenged by a number of requirements: i) efficient channeling of photogenerated electrons and holes to redox reactions at the interfaces; ii) efficient absorption of sunlight; iii) avoidance of photo-oxidation of the semiconductor electrode. For example, although the first photolytic cell was demonstrated with titania, which meets the first and last requirements, it can absorb only the ultraviolet portion of the sunlight due to its large band gap (i.e., 3.0 eV for rutile). On the other hand, silicon meets requirements (i) and (ii), but it undergoes photo-oxidation.
What is needed is a system and method for addressing the above and related issues.